Visual inspection of parts or structures for defects typically requires good spatial resolution to see the defects, but may also require a large focus range. But to obtain the best resolution from an imaging system, it needs to have a low f-number which limits the usable depth of field. Methods to use autofocus or focus stacking provides more range at high resolution, but often at the expense of computation time, loss of a real time image and uncertainty in scale changes. This paper describes an approach to quickly move through a range of focus positions without the need to move optics mechanically in a manner that is highly repeatable, maintains high resolution and provides the potential for a live image directly viewable by an inspector, even at microscope level magnifications. This paper will present the approach we investigated and discuss the pros and cons for a range of applications from large structures to small feature inspection. The paper will present examples of what resolution was achieved and how the multiple images might also be used to determine other parameters such as pose of a test surface.

Date Published: 19 May 2016
PDF: 8 pages
Proc. SPIE 9868, Dimensional Optical Metrology and Inspection for Practical Applications V, 98680D (19 May 2016); doi: 10.1117/12.2224604

Published in SPIE Proceedings Vol. 9868:
Dimensional Optical Metrology and Inspection for Practical Applications V
Kevin G. Harding; Song Zhang, Editor(s)